Electrode structure with titanium alloy base



4 J. B. COTTON ETAL 3,118,828

ELECTRODE STRUCTURE WITH TITANIUM ALLOY BASE Filed Jan. 25, 1960 6 Sheets-Sheet l w m M 0 A w w u U m Z a N mm "M E gas/122v F553 3 n 09 0m m n 0Q 0m 0w m m 0. 0 W M 4 T w M W a o i b 3 b u a K 1 J M 23.2(FFUN5Q 0N hjuuuiiou M n M m O W l on W ow M Q. W $55.02 0m 23 225. n 00. O: ION- roam 5 r5 Jan. 21, 1964 J. B. COTTON ETAL 3,118,828

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ELECTRODE STRUCTURE WITH TITANIUM ALLOY BASE Filed Jan. '25, 1960 6 Sheets-Sheet 5 5 I w m m m r v Q M a M m m GE M A ma z =zv pzmuusu 3 oo 0m 0m OF 0G 00 0v ON ON O o M 5 J m $926.; $50. 53052200 W LI 0 in R 0. Q 9 z uJuz w x x1 x x x o m. ZwZ 12 512% u D T T U D D 0 WM w wz wd2 m United States Patent 3,118,828 ELECTRODE STRUCTURE WITH TITANIUM ALLOY BASE Joseph Bernard Qotton, Sutton Coldfield, and Marion Katharine McQuillan, Birmingham, England, assignors to imperial Qhemical Industries Limited, London, England, a corporation of Great Britain Filed .l'an. 25, 1960, Ser. No. 4,375 (Ilairns priority, application Great Britain Jan. 28, 1959 4 Claims. (Cl. 204290) This invention is an improvement in or modification of the invention described in co-pending US. application Serial No. 748,938, filed July 16, 1958, and Serial No. 791,070, now abandoned, filed February 4, 1959.

In the said US. applications an electrode structure is described and claimed which comprises a support of which the surface consists of titanium or a titanium-base alloy having auodic polarisation properties comparable to those of pure titanium and having in contact with part or all of the said surface a platinum group metal or alloy constituted by platinum group metals. The metal may be platinum or rhodium, and the alloy constituted by platinum group metals may contain platinum or rhodium.

The present invention is concerned with such electrode structures in which only part of the surface is in contact with the platinum group metal or alloy.

In accordance with the said invention an electrode structure includes a support of which part of the surface is in contact with platinum, rhodium or an alloy comprising platinum or rhodium, and of which at least part of the s rface which is not in said contact consists of an alloy having a titanium base alloyed with niobium or tantalum, the niobium or tantalum being present in the amount of more than 1% by weight, heat-treated in such a way that the phase structure of said alloy is such that its beta phase content at the heat-treatment temperature is at least 10% by volume.

On cooling from the heat-treatment temperature the beta phase content, depending upon the composition, may partly or wholly decompose, the product of the decomposition being hereinafter called transformed beta.

in accordance with another aspect of the invention an alloy having a higher breakdown figure (as hereinafter defined) than commercially pure titanium comprises beycen 2! 2% and 10% by weight of niobium, balance titanium and impurities, having a retained or transformed beta phase content of at least 30% by volume.

in accordance with a further aspect of the invention an alloy having a higher breakdown figure than commercially pure titanium comprises between 5% and 20% by weight of tantalum, balance titanium and impurities, having a retained or transformed beta phase content of at least 30% by volume.

Whilst an increase in the niobium or tantalum content enables increased percentages of beta phase to be present, the cost of niobium and tantalum makes it desirable to use minimum percentages of these constituents. Furthermore the effect of tantalum is such that twice as much of this is needed as of niobium to obtain similar results, and when less than 2% of niobium is present, the necessary heat-treatment to ensure adequate beta content is more difficult; for these reasons the preferred alloying constituent is niobium; the preferred content is about 2%.% of this constituent and the beta phase content (to give a substantial advantage) is preferably at least 30%.

The beta phase content may be retained beta, ransformed beta or a mixture of the two.

The invention will now be more particularly described with reference to the accompanying drawings wherein ice FIGURES 1 to 6 are graphs showing polarisation curves of different alloys and metals.

It is known that when a potential is: imposedupona, titanium electrode the electrode polariscs and'little or;

no current flows until the voltage exceeds a figure hereinrefelred to as the breakdown figure. When the voltage exceeds this figure current flows and the electrode corrodes. However, by coating part of the electrode with for example platinum the electrode will pass current at lower voltages. In such cases, the uncoated portion of the electrode does not pass any sensible amount of current and does not corrode providing the voltage is below the breakdown figure.

Whilst the precise level of the breakdown figure varies according to the operating conditions it is in the region of 12 volts for commercially pure titanium in chloride solutions, and as high as -l00 volts in sulphuric and phosphoric acid solutions. FIGURE 1 illustrates by way of comparison the polarisation curves for titanium, niobium and tantalum in a 5% solution of sodium chloride.

The limitation on voltage which may be impressed without corrosion of the electrode is disadvantageous for certain uses, for example where the size of the coated portion is limited, where the uncoated portion or a part thereof must be immersed in electrolyte and specific current densities are required which necessitate voltages higher than the breakdown figure. One such case is in the internal cathodic protection of large diameter pipe lines where in a specific instance a rod some 3 feet long is used as the electrode and only its tip is plated with platinum. To obtain satisfactory current densities with such a rod a voltage higher than the breakdown figure may be necessary.

Examination of the polarisation curves of niobium and tantalum (FIGURE 1) indicates that alloys of niobium or tantalum with titanium may yield polarisation curves which would enable higher potentials to be used without exceeding the breakdown figure for the alloy but experiment has proved that this is not necessarily the case unless an uneconomic percentage of niobium or tantalum is used. In any event, the theory that mere alloying of titanium with niobium or tantalum will yield higher polarisation curves than titanium alone has as its obvious corollary that better results would be obtained with higher percentages of these alloying constituents and experiment has shown that in the absence of the beta phase this is not necessarily true. As niobium and tantalum are more expensive than titanium (which prevents their economic use as substitutes) it might be thought that the problem of finding an economically feasible alloy for a specific potential was merely a matter of determining the minimum effective percentage of alloying constituent.

Whilst it remains important to determine the minimum effective percentage of alloying constituents because this has an important bearing on the cost of the electrodes, experiment has proved that the percentage of alloying constituent is only one of two critical factors.

FEGURE 2 shows the results of an experiment in which three examples of an alloy of titanium and 5% niobium were tested to determine their breakdown figures. For the purpose of comparison the polarisation curve of a commercially pure titanium sample is also shown in this figure, as it is on each of the other figures. Each of the samples was used as an anode approximately 0.05 in. x 1.5 in. x 0.6 in, ground to give a flat surface, activated in aqueous pickle containing 5% hydrofluoric acid and 20% nitric acid before blanking with a lacquer to leave approximately half a square inch of metal. The samples were subjected to anodic DC. current whilst submerged in an electrolyte of a 5% solution of sodium chloride, and using a cathode of commercially pure titanium measuring 2 in. x .5 in. x 0.05 in. placed 1.5 in. from the sample. The graph curves were constructed by raising the applied current incrementally and taking values of the potential difference and current at approximately 3 minute intervals.

Samples Nos. 1 and 3 were taken from different batches of alloy which was formed into the anode without any treatment other than that mentioned above. Sample No. 2 which gave a substantially higher value for its breakdown figure was treated by heating for 30 minutes at 800 C. and then water quenching.

The purpose of the heat-treatment was to give an alpha plus beta phase structure containing between 30 and 40% of retained or transformed beta. Microscopic examination and X-ray analysis confirmed that the beta content of this sample was about 33% FIGURE 3 shows the results of an experiment on three further samples of the same alloy, that is 95% titanium and niobium, fabricated and tested under the same conditions as Samples Nos. 1-3, contrasted with the polarisation curve of commercially pure titanium. Sample No. 4 was treated at 700 C. for 30 minutes, No. 5 at 800 C. for the same time and No. 6 at 900 C. for the same time, and in each case the samples were water quenched.

It will be noted that Sample No. 6 produced higher breakdown figures at low current values, that Sample No. 4 produced the highest breakdown figure at intermediate current values, that is 3060 milliamps. but that No. 5 had the best overall performance.

Experiments were also made to determine the effect of varying the alloying constituent, and to this end three further samples fabricated in a similar manner and tested under the same conditions as Samples Nos. 1-6, containing in the case of No. 7, niobium and 90% titanium, No. 8, 4.32% niobium, balance titanium and No.9, 2.54% niobium, balance titanium, were tested. Sample No. 7 was heat-treated at 700 C., No. 8 at 790 C. and No. 9 at 820 C. in each case for 30 minutes and the samples were water quenched. These heat-treatments were designed to yield beta contents of about 33% and the temperature of the treatment varied in approximately inverse proportion to the quantity of alloying constituent.

The results of these further experiments are illustrated in FIGURE 4. As will be seen, the breakdown figure is substantially uniform in the three samples at a current of 100 milliamps. although the alloy richer in niobium was better at lower current densities. In order to ascertain the minimum effective quantity of niobium further tests were made on Samples Nos. 10, 11, 12 and 13, all fabricated and tested in the same way as Samples Nos. 1-9,

containing 0.01 0.5%, 1% and 5% of niobium, balance titanium respectively. Sample No. 10 was tested as rolled, Nos. 11 and 12 after heating for 30 minutes at 870 C. and water quenching, and No. 13 after heating for 30 minutes at 800 C. and water quenching. The results of these tests are shown in FIGURE 5.

It will be seen that the improvement in breakdown figure over and above that for commercially pure titanium is in these alloys substantially proportional to the niobium percentage. However, it is believed that this is largely because the heat-treatment to obtain specific transformed beta percentages becomes more and more critical at lower niobium percentages (at which there is little or no retained beta content) and that small variations from the calculated heat-treatment temperature will lead to considerable changes in structure and in breakdown figures. The smallest percentage of niobium which yields substantial results in combination with heat-treatment within the limits of accuracy possible during the experiments appears to be about 2.5%. However the experiments indicate that any percentage of niobium will yield some advantage, however small, and that satisfactory advantages can be obtained with niobium contents lower than '4 2.5% if the heat-treatment is controlled within fine limits.

FIGURE 6 illustrates the results of tests with alloys containing tantalum showing graphs of results on tests with Samples Nos. 14, 15 and 16 containing respectively 5.3%, 0.1% and 0.01% of tantalum, balance titanium, all of which samples were fabricated and tested in the same way as Samples Nos. l-13. Sample No. 14 was heat-treated at 775 C. for 30 minutes and then water quenched, but the other samples were not heat-treated.

It is believed that the effect of tantalum in larger quantities will produce results comparable to that of niobium, providing heat-treatment to ensure adequate beta phase content is carried out. However the percentage of tantalum should be about twice that of niobium to give similar results.

The theory on which the invention is based therefore is that the niobiu'm or tantalum is effective in two ways in relation to the breakdown figure; firstly by its effect as might be expected from an examination of FIGURE 1, and secondly, when accompanied by suitable heat-treatment in enabling the beta phase to be present, that is as a beta forming or stabilising agent, because the presence of niobium or tantalum in an alloy of titanium in which the beta phase of the latter has been formed by heattreatment gives an improvement in the breakdown figure far above what would be expected from examination of FIGURE 1 and far above what can be realised with the same alloy in the absence of such heat-treatment.

Further experiments have shown that beta formation itself is ineffective when niobium or tantalum is absent. These further experiments included making a series of titanium-molybdenum alloys having the desired phase structure and such alloys were found to have similar breakdown figures to commercially pure titanium.

We claim:

1. An electrode comprising an element having a surface which is partly in contact with a metallic material constituting the active surface of said electrode, said material being selected from the class consisting of platinum group metals and alloys consisting of these metals, at least part of the surface of said element which is not in such contact being an alloy consisting essentially of a titanium base alloyed with a metal selected from the group consisting of niobium and tantalum, said niobium being present in amounts of about 2.5 to 10 weight percent, said tantalum being present in amounts of about 5 to 20 weight percent and said alloy being heat treated at a temperature from 700 to 900 C. and subsequently cooled, said heat treatment being continued for a time sufficient to substantially increase the breakdown voltage of said electrode.

2. An electrode according to claim 1 wherein said active surface material is platinum.

3. An electrode according to claim 1 wherein the alloying constituent is niobium.

4. An electrode according to claim 3 wherein the niobium is present in amounts of about 2.5 to 5% by weight.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES 82Corrosi0n Technol gy, arch 1 p g item CT 

1. AN ELECTRODE COMPRISING AN ELEMENT HAVING A SURFACE WHICH IS PARTLY IN CONTACT WITH A METALLIC MATERIAL CONSTITUTING THE ACTIVE SURFACE OF SAID ELECTRODE, SAID MATERIAL BEING SELECTED FROM THE CLASSCONSISTING OF PLATNUM GROUP METALS AND ALLOYS CONSISTING OF THESE METALS AT LEAST PART OF THE SURFACE OF SAID ELEMENT WHICH IS NOT IN SUCH CONTACT BEING AN ALLOY CONSISTING ESSENTIALLY OF A TITANIUM BASE ALLOYED WITH A METAL SELECTED FROM THE GROUP CONSISTING OF NIOBIUM AND TANTALUM, SAID NIOBIUM BEING PRESENT IN AMOUNTS OF ABOUT 2.5 TO 10 WEIGHT PERCENT, SAID TANTALUM BEING PRESENT IN AMOUNTS OF ABOUT 5 TO 20 WEIGHT PERCENT AND SAID ALLOY BEING HEAT TREATED AT A TEMPERATURE FROM 700* TO 900*C. AND SUBSEQUENTLY COOLED, SAID HEAT TREATMENT BEING CONTINUED FOR A TIME SUFFICIENT TO SUBSTANTIALLY INCREASER THE BREAKDOWN VOLTAGE OF SAID ELECTRODE. 